Receiving devices that receive a radio signal of a radio frequency band with a radio frequency received through an antenna as a center have been known. For example, this type of receiving device includes a local oscillator that generates a local signal of a local frequency different from a radio frequency and a mixer that converts a radio signal received through an antenna into an intermediate signal of an intermediate frequency band based on the local signal. The receiving device further includes a filter that attenuates a signal outside the intermediate frequency band in the signal converted by the mixer at a stage subsequent to the mixer.
There are cases in which the antenna receives an interference wave of a band near the radio signal together with the radio signal. In this case, the mixer converts the radio signal into an intermediate signal IS1, and converts the interference wave into an interference wave IW1 of a band near the intermediate frequency band as illustrated in FIG. 1 (A). However, the filter gradually decreases in transmissivity as it gets away from the intermediate frequency band as indicated by an alternate long and short dash line FL1. Here, the transmissivity refers to a ratio of a level of a signal output from the filter to a level of a signal input to the filter.
Thus, it is difficult for the filter to sufficiently attenuate a signal of a band FW1 which is outside the intermediate frequency band but near the intermediate frequency band. As a result, the intermediate interference wave IW1 passes through the filter without being sufficiently attenuated.
In this regard, for example, a technique in which a receiving device detects a band of an intermediate interference wave, and changes a local frequency based on the detected band so that an intermediate interference wave IW1 has a band FW2 which is sufficiently attenuated by the filter as illustrated in FIG. 1 (B) (e.g., see Japanese Laid-open Patent Publication No. 11-186923). As a result, the intermediate interference wave IW1 can be sufficiently attenuated by the filter.
Further, notch filters that attenuate a signal within a certain removal band have been known (e.g., see Japanese Laid-open Patent Publication No. 04-360441 and Japanese Laid-open Patent Publication No. 2004-336715).
Meanwhile, for example, there are cases in which the interference wave within the intermediate frequency band received by the antenna reaches the side subsequent to the mixer without undergoing frequency conversion by the mixer, or cases in which the interference wave within the intermediate frequency band comes into the receiving device at the side subsequent to the mixer, for example, due to an insufficient separation (isolation) between two signals input to the mixer or characteristics of circuit elements. In this case, it is difficult to change the band of the interference wave even when the local frequency is changed as in the above-described receiving device. Particularly, when a housing made of a material such as synthetic resin is used in order to lighten the receiving device, the interference wave is hardly attenuated due to the housing, and thus this problem becomes more remarkable.
In this regard, the interference wave is considered to be sufficiently attenuated using the notch filter. In this case, however, since the intermediate signal as well as the interference wave is attenuated, the quality of a reception signal deteriorates. In this manner, the above-described receiving device has a problem in that the quality of a reception signal deteriorates.